Difficulty: Intermediate | Prerequisites: Stereochemistry fundamentals (Part 1), single and double bond basics, Lewis structures
Big picture: this section covers two related ideas. First, how the rotation around single bonds creates different three-dimensional shapes (conformations) with different energies, and which shapes are preferred. Second, how addition reactions to double and triple bonds deliver atoms from the same face (syn) or opposite faces (anti), and how the choice of reagent controls the stereochemical outcome. Together, these ideas let you predict the geometry of reaction products and understand why certain reagents give specific stereoisomers.
Conformational analysis is about which rotational arrangements around single bonds are most stable and why. Reaction stereochemistry is about whether atoms add to the same face (syn) or opposite faces (anti) of a double or triple bond, and which reagents give which outcome. Knowing both lets you predict the three-dimensional shape of a product before you draw it.
Newman projection
A way of viewing a molecule along a C-C bond axis. The front carbon is drawn as a dot, the back carbon as a circle. Substituents radiate outward from each. This projection makes it easy to see the dihedral angle between groups and assess strain.
Staggered conformation
A rotational arrangement where substituents on adjacent carbons are as far apart as possible (60-degree dihedral angles between them in a Newman projection). Lower energy than eclipsed.
Eclipsed conformation
A rotational arrangement where substituents on adjacent carbons are aligned directly behind one another (0-degree dihedral angle). Higher energy because of torsional strain.
Anti conformation
The staggered arrangement where the two largest substituents are 180 degrees apart (on opposite sides of the Newman projection). The most stable conformation of n-butane.
Gauche conformation
The staggered arrangement where the two largest substituents are 60 degrees apart. More stable than eclipsed, but less stable than anti because of steric strain between the nearby bulky groups. About 3.8 kJ/mol higher in energy than anti for n-butane.
Torsional strain
The energy cost of eclipsed bonds, caused by repulsion between bonding electrons on adjacent atoms that are forced into close proximity.
Steric strain (van der Waals strain)
The energy cost of bulky groups being pushed too close together. This is what makes gauche less stable than anti, and what makes the fully eclipsed conformation of butane (where the two methyl groups overlap) the highest-energy arrangement.
Syn addition
Both new atoms or groups add to the same face of a double bond. Catalytic hydrogenation (H2 with Pt, Pd, or Ni) is the classic example: both hydrogens are delivered from the metal surface to the same side.
Anti addition
The two new atoms or groups add to opposite faces of the double bond. Dissolving metal reduction (Na/NH3) of alkynes gives anti addition, producing the trans alkene.
Lindlar catalyst
A "poisoned" palladium catalyst (Pd on CaCO3, treated with lead acetate and quinoline) that reduces alkynes to cis alkenes and stops there. It delivers H2 by syn addition and its reduced activity prevents over-reduction to the alkane. Think of it as the gentle version of catalytic hydrogenation.
Dissolving metal reduction (Birch-type conditions)
Reduction of alkynes using an alkali metal (Na or Li) in liquid ammonia. Produces the trans alkene via an anti addition mechanism involving radical anion intermediates. The mechanism is stepwise (not surface-based), which is why the stereochemical outcome differs from catalytic hydrogenation.
As you rotate around the C2-C3 bond of n-butane, the molecule passes through a repeating cycle of staggered and eclipsed conformations.
Stability ranking (most stable to least stable): anti > gauche > eclipsed (H vs CH3) > fully eclipsed (CH3 vs CH3).
Anti is the global energy minimum. The two methyl groups are 180 degrees apart, minimising steric strain.
Gauche is a local energy minimum. The two methyl groups are 60 degrees apart, introducing some steric strain (about 3.8 kJ/mol above anti).
Eclipsed conformations are energy maxima (transition states during rotation). The worst case is the fully eclipsed form where both methyl groups overlap, producing maximum torsional and steric strain.
At room temperature, butane molecules freely interconvert between conformations, spending the most time in the anti form. Anti and gauche butane are the same compound, not different isomers.
H2 with a metal catalyst (Pt, Pd, or Ni) adds both hydrogen atoms to the same face of a double bond.
The mechanism: both the alkene and H2 adsorb onto the metal surface. The two hydrogen atoms are delivered simultaneously from the surface side, so they end up on the same face. This is syn addition.
The product retains the spatial relationship imposed by having both H atoms added from one side.
The Lindlar catalyst (Pd/CaCO3, poisoned with lead acetate and quinoline) reduces an alkyne to an alkene and stops.
Because it is still a surface-catalysed hydrogenation, the addition is syn, placing both new hydrogen atoms on the same side of the resulting double bond.
Product: the cis (Z) alkene. For example, propyne + H2/Lindlar gives cis-propene (propene).
Na (or Li) in liquid ammonia (NH3) reduces an alkyne to an alkene, but by a completely different mechanism from catalytic hydrogenation.
The mechanism involves sequential single-electron transfers and protonations. A radical anion intermediate forms, and the geometry of the intermediate favours the trans arrangement of substituents to minimise steric interactions.
Product: the trans (E) alkene. For example, 2-butyne + Na/NH3 gives trans-2-butene.
Propyne (CH3-C-triple bond-CH) has a methyl group on one end and a hydrogen on the other.
When reduced to propene (CH3-CH=CH2), both the cis and trans labels become meaningless: there is only one possible geometric isomer of propene, because one end of the double bond carries two identical substituents (two H atoms on the terminal carbon).
You need at least two different substituents on each carbon of the double bond to distinguish cis from trans. 2-Butyne (CH3-C-triple bond-C-CH3) works because both ends carry a methyl and a hydrogen after reduction.
This is why propyne is unsuitable for testing whether a reduction is syn-selective or anti-selective.
If a reaction gives exclusively the trans alkene from an alkyne, the addition must be anti (atoms delivered to opposite faces).
Catalytic hydrogenation of alkenes gives syn addition. If the alkyne reduction gives anti addition, the mechanism cannot be a simple surface-catalysed process.
Conclusion: the alkyne-to-trans-alkene reduction (Na/NH3) proceeds by a fundamentally different mechanism from catalytic hydrogenation, involving stepwise electron transfer in solution rather than simultaneous delivery from a metal surface.
Conformational analysis matters in drug design, where the preferred three-dimensional shape of a molecule determines how well it fits an enzyme active site or receptor. Syn vs anti selectivity in alkyne reduction is used routinely in organic synthesis to build molecules with precisely controlled geometry, for example in the synthesis of pheromones, fatty acids, and pharmaceutical intermediates.
Students sometimes think the gauche conformation is unstable enough that it never exists. In fact, gauche is a local minimum and butane spends a meaningful fraction of its time there at room temperature. It is less stable than anti, but it is still a real, populated conformation.
A common error is assuming that Lindlar and Na/NH3 give the same product. They give opposite stereochemical outcomes: Lindlar gives cis (syn addition), Na/NH3 gives trans (anti addition). Mixing these up is one of the easiest ways to lose marks.
Students often try to test stereoselectivity using propyne. Because propene has no cis/trans isomerism, propyne cannot distinguish syn from anti addition. Always use a symmetric internal alkyne (like 2-butyne) for this purpose.
Some students assume that all hydrogenation reactions proceed by syn addition. Catalytic hydrogenation does, but dissolving metal reduction is not catalytic hydrogenation, and it proceeds by a different mechanism with anti selectivity.
Expect a question asking you to compare the stability of anti vs gauche butane and explain why. Know the energy difference and cite steric strain.
Predicting the product of alkyne reductions is a staple. Remember: Lindlar = cis, Na/NH3 = trans.
Questions asking why propyne is unsuitable for testing stereoselectivity test whether you understand the requirement for cis/trans isomerism in the product.
Mechanistic reasoning questions may ask you to deduce the mechanism type (surface vs solution) from the stereochemical outcome. If the product is trans from an alkyne, the mechanism cannot be a simple surface hydrogenation.
Conformer vs isomer distinction: if a question asks for the relationship between anti and gauche butane, the answer is conformers (identical), not diastereomers or constitutional isomers.
True or false: the gauche conformation of n-butane is an energy maximum. (False. Gauche is a local energy minimum; eclipsed conformations are the maxima.)
Fill in the blank: catalytic hydrogenation of an alkene with H2/Pt proceeds by ____ addition. (Syn.)
True or false: reducing 2-butyne with a Lindlar catalyst gives trans-2-butene. (False. Lindlar gives the cis alkene.)
Fill in the blank: propyne is unsuitable for testing stereoselectivity because its reduction product, propene, cannot show ____ isomerism. (Cis/trans, or geometric.)
True or false: if an alkyne reduction gives exclusively the trans alkene, the mechanism must involve anti addition. (True.)
Q: Compare the stability of the anti and gauche conformations of n-butane. Which is more stable and why?
A: Anti is more stable. In the anti conformation the two methyl groups are 180 degrees apart, minimising steric strain. In gauche they are only 60 degrees apart, and the repulsion between them raises the energy by about 3.8 kJ/mol.
Q: Predict the major product of reducing 2-butyne with Na in liquid ammonia (Na/NH3). What is the stereochemistry?
A: Trans-2-butene. The dissolving metal reduction proceeds by anti addition, placing the two hydrogen atoms on opposite faces of the new double bond.
Q: Predict the product of reducing propyne with H2 and a Lindlar catalyst.
A: Propene (CH3CH=CH2). The Lindlar catalyst gives syn addition (cis alkene), but since propene has no cis/trans isomerism (the terminal carbon bears two identical H atoms), only one geometric product is possible.
Q: Explain why propyne is unsuitable for determining whether a reduction is cis-selective or trans-selective.
A: The product, propene, has two identical substituents (H atoms) on the terminal carbon. Without two different groups on each end of the double bond, there is no cis/trans distinction to observe. You need an internal alkyne like 2-butyne, where each end of the resulting double bond has one methyl and one hydrogen.
Q: A reduction of an alkyne gives exclusively the trans alkene, but catalytic hydrogenation (H2/Pt) of alkenes gives syn addition. What can you conclude about the alkyne reduction mechanism?
A: The alkyne reduction must proceed by a different mechanism from catalytic hydrogenation. Since catalytic hydrogenation is syn (same-face delivery from a metal surface), and this reaction gives the trans (anti) product, the alkyne reduction must involve anti addition through a stepwise, solution-phase mechanism rather than surface-catalysed delivery.
Q: What is the relationship between anti butane and gauche butane?
A: They are conformers (also called conformational isomers). They are the same compound in different rotational arrangements around the C2-C3 bond and interconvert freely at room temperature.
Conformational analysis feeds directly into cyclohexane chemistry, where axial vs equatorial preferences depend on the same steric arguments as anti vs gauche. Syn and anti addition stereochemistry reappears in electrophilic addition to alkenes (e.g., halogenation, hydroboration), where the mechanism determines the facial selectivity. These concepts also connect to stereochemistry fundamentals (Part 1 of these notes): knowing enantiomer and diastereomer relationships lets you predict whether a syn or anti addition to an alkene with existing chiral centres will produce one product or a mixture.
Conformational analysis, Newman projection, anti conformation, gauche conformation, staggered, eclipsed, torsional strain, steric strain, van der Waals strain, energy diagram, dihedral angle, n-butane conformations, syn addition, anti addition, catalytic hydrogenation, H2/Pt, H2/Pd, Lindlar catalyst, poisoned palladium, cis alkene, trans alkene, dissolving metal reduction, Na/NH3, Birch reduction, alkyne reduction, 2-butyne, propyne, stereoselectivity, cis-trans isomerism, geometric isomerism, E/Z configuration, organic chemistry, UW, University of Washington